Locating Fuel tank

[Nasher]

Guys

The Under deck 130l stainless steel fuel tank in my - Cough - RIB, has come loose and is moving around @1/2 inch in all directions.

I believe it was originally fixed in place using expanding foam around the sides etc before the deck was glassed in over the top.

I face the prospect of major surgery to cut the seats and deck out to dig out the old foam and do the job properly, or drill some holes to get a nozzle in and squirt some more foam in down the sides between the tank and longitudinal frames.

Being lazy and still wanting to use it this year the second option is my preferred method.

So is there a particular foam product I should be using?
I assume a closed cell is preferable to open cell so it doesn't soak up stray water/fuel?
Is there a fuel proof product?
Am I on the wrong track completely?

Thanks

Nasher.